In magneto-optical recording, a portion of a magnetic film of a magneto-optical recording medium is locally heated to a Curie point or compensation composition temperature or higher by irradiation of laser light. The heated portion is magnetized in the direction of an external magnetic field, thereby forming a recording magnetic domain where an information signal is recorded. The magnetic film onto which the information signal is recorded is also referred to as a recording magnetic film (or simply recording film).
Among such magneto-optical recording methods for the magneto-optical recording medium is a magnetic field modulation recording method. In this method, the temperature of an overall recording magnetic film is increased by irradiation of laser light. An external magnetic field having a modulated direction in accordance with a recording signal is applied to a given portion of the recording magnetic film. The recording signal is thermomagnetically recorded on the given portion. This is called a magnetic field modulation recording method. Alternatively, laser light having a modulated intensity in accordance with a recording signal is irradiated onto a given portion of a recording magnetic film. The temperature of the given portion is increased so that the recording signal is thermomagnetically recorded onto the given portion. This is called an optical modulation recording method.
In a conventional magneto-optical recording medium, when the size of the recording magnetic domain is smaller than or equal to the diameter of a reproducing light spot, recording magnetic domains at the front and rear side of the recording magnetic domain which is a target to be reproduced are included in the reproducing light spot (i.e., a detection range). Interference of the adjacent recording magnetic domains causes a decrease in the reproduced signal, whereby the S/N ratio is reduced or the reproduction signal is not output.
A magneto-optical recording and reproducing method using magnetic super-resolution as shown in FIGS. 1A and 1B is a proposed technique to solve such a problem (see Nikkei Electronics, No. 539, Oct. 28, 1991). This magneto-optical recording and reproducing method will be briefly described below.
As shown in a cross-sectional view of FIG. 1B, a magneto-optical recording medium 60 includes a reproducing magnetic film 63, a transcribing magnetic film 64A, an intermediate film 64, and a recording magnetic film 65 which are successively provided on a substrate (not shown). An arrow X in FIG. 1B indicates a moving direction along a track of the magneto-optical recording medium 60. An upward arrow 61 indicates a magnetic field for recording and reproduction. A downward arrow 62 indicates an initial magnetic field.
FIG. 1A is a plan view illustrating a part of a track of the magneto-optical recording medium 60.
As shown in FIGS. 1A and 1B, when reproducing information, a reproducing light spot 67 is formed along the track. When laser light is irradiated onto the rotating magneto-optical recording medium 60, the temperature distribution of a magnetic film structure including the reproducing magnetic film 63 and the transcribing magnetic film 64A are not rotation symmetrical around the center of the circular reproducing light spot 67. Specifically, a region 70 which has been irradiated by the reproducing light spot 67 has a high temperature greater than or equal to the Curie temperature Tc of the transcribing magnetic film 64A (the region 70 is referred to as a high temperature region 70). A crescent-shaped region 72, which is positioned at the left side of the reproducing light spot 67 and outside the high temperature region 70, has an intermediate temperature (the region 72 is referred to as an intermediate temperature region 72). A region 71 which is positioned at the right side of the intermediate temperature region 72 and within the reproducing light spot 67 has a low temperature (the region 71 is referred to as a low temperature region 71).
Assuming that a signal (information) is already thermomagnetically recorded as a recording magnetic domain 69 on the recording magnetic film 65, the transcribing magnetic film 64A is strongly exchange-coupled with the reproducing magnetic film 63. The intermediate film 64 is provided in such a manner that the magnetic domain wall becomes stable when the magnetization direction of the reproducing magnetic film 63 is in agreement with the magnetization direction of the recording magnetic film 65.
The reproducing operation of the magneto-optical recording medium 60 thus constructed will be described below.
The reproducing magnetic film 63 initially has the same magnetization direction as that of the initializing magnetic field 62. Upon reproduction, laser light for reproduction is irradiated to a range between X1 and X2 shown in FIG. 1B. The laser light forms the reproducing light spot 67 on the rotating magneto-optical recording medium 60. This causes an increase in temperature of the rotating magneto-optical recording medium 60, resulting in a temperature distribution shown in FIG. 1A (i.e., the temperature region 70, 71, and 72). The coercive force of the reproducing magnetic film 63 is decreased due to the temperature increase. Exchange-coupling with the recording magnetic film 65 is therefore dominant in the intermediate temperature region 72, so that the magnetization of the reproducing magnetic film 63 is directed to the magnetization direction of the recording magnetic film 65.
In the high temperature region 70 having a temperature of Tc or higher, the magnetization of the transcribing magnetic film 64A disappears in some portions thereof. Exchange-coupling between the reproducing magnetic film 63 and the recording magnetic film 65 is cut off at these portions, so that the magnetization of the reproducing magnetic film 63 is directed to the magnetization direction of the reproducing magnetic field 61. Accordingly, the low and high temperature regions 71 and 70 within the reproducing light spot 67 masks the recording magnetic domains 69. Only from a recording magnetic domain 69X positioned in the intermediate temperature region 72 is information read as a reproduced signal.
With the above-described method, even when a single recording magnetic domain 69 has a size smaller than the diameter of the reproducing light spot 67, there occurs substantially no interference by recording magnetic domains 69 ahead of and behind the single recording magnetic domain 69. It is therefore possible to reproduce information stored in high density.
There is, however, a drawback with the above-described magneto-optical recording medium 60 as it needs the initializing magnetic field 62 for initially directing the magnetization of the reproducing magnetic film 63 in a single direction.
Japanese Laid-Open Publication No. 5-81717 proposes a magneto-optical recording medium 80 having a structure shown in FIGS. 2A and 2B which does not need the initializing magnetic field.
As shown in a cross-sectional view of FIG. 2B, the magneto-optical recording medium 80 includes a reproducing magnetic film 83 and a recording magnetic film 85 on a substrate (not shown). An arrow X represents a moving direction along a track of the magneto-optical recording medium 80. As is different from the magneto-optical recording medium 60 shown in FIGS. 1A and 1B, an in-plane magnetization film is used as the reproducing magnetic film 83 in the magneto-optical recording medium 80.
FIG. 2A is a plan view illustrating part of the track of the magneto-optical recording medium 80. Similar to the magneto-optical recording medium 60 described with reference to FIGS. 1A and 1B, laser light is irradiated in a range between X1 and X2 along the track of FIG. 2B upon reproduction. The laser light forms a reproducing light spot 87. When laser light is irradiated onto the rotating magneto-optical recording medium 80, the temperature distributions of a reproducing magnetic film 83 and a transcribing magnetic film 85 are not rotation symmetrical around the center of the circular reproducing light spot 87. Specifically, a region which has been irradiated by the reproducing light spot 87 and is currently irradiated by a left-end portion of the reproducing light spot 87 forms a high temperature region 90. A region which is included in the reproducing light spot 87 and outside the high temperature region 90 forms a low temperature region 91. Also in this case, a recording magnetic domain 89 is smaller than the reproducing light spot 87.
The reproducing operation of the magneto-optical recording medium 80 thus constructed will be described below.
Assuming that a recording signal has been previously recorded in the recording magnetic domains 89 of the recording magnetic film 85 by the thermomagnetically recording, the reproducing magnetic film 83 is an in-plane magnetization film having a magnetic anisotropy in an in-plane direction parallel to the film at room temperature. Only the high temperature region 90 within the reproducing light spot 87 of the reproducing magnetic film 83 is a vertical magnetization film having a magnetic anisotropy in a direction perpendicular to the film. When laser light for reproduction is irradiated onto a range between X1 and X2 shown in FIG. 2B, the temperature of the magneto-optical recording medium 80 is increased so that the high temperature region 90 and the low temperature region 91 are formed. In the high temperature region 90, the reproducing magnetic film 83 is changed to the vertical magnetization film, and is exchange-coupled with the recording magnetic film 85 so that the magnetization of the reproducing magnetic film 83 is directed to the magnetization direction of the recording magnetic film 85. When the magneto-optical recording medium 80 is moved in the X direction so that the temperature of the magneto-optical recording medium 80 is decreased, the reproducing magnetic film 83 is changed to an in-plane magnetization film.
In the magneto-optical recording medium 80, information stored in the recording magnetic domains 89 which are smaller than the reproducing light spot 87 can thus be reproduced without the initializing magnetic field.
In the magneto-optical recording medium 80, when the reproducing magnetic film 83 includes an in-plane magnetization film, the initialized magnetization field is not necessary. However, there are the following drawbacks.
The magnetization direction of the reproducing magnetic film 83 is attracted toward the recording magnetic film 85 due to magnetic coupling between the reproducing magnetic film 83 and the recording magnetic film 85. For this reason, the magnetization direction of the reproducing magnetic film 83 is not held in an ideal in-plane magnetization direction but has a vertical component of magnetization in the low temperature region 91 even within the light spot. As a result, transcription occurs even in a region which does not need transcription of the recording magnetic domain 89. This leads to insufficient resolution upon reproduction or occurrence of noise upon transcription.
Further, the critical temperature of the reproducing magnetic 83 at which it changes from an in-plane magnetization film to a vertical magnetization film is constant. For this reason, as the reproducing power of a laser beam for reproduction is changed, the region where the recording magnetic domain 89 is transcribed is changed, whereby waveform interference degrades the reproduction characteristic.
Furthermore, as a magneto-optical recording medium having a high resolution and a high-performance reproduction characteristic without the need for an initializing magnetic field, there is a magneto-optical recording medium having a reproducing magnetic film of a shrink type (magnetic domain wall shrink type). Assuming that a recording signal is read only from a particular temperature region within the reproducing light spot, the use of this shrink type reproducing magnetic film leads to an unstable shrink operation in the arrangement including only the recording magnetic film and the reproducing magnetic film. To address this, the decreased magnetic coupling force may allow stabilization of the shrink operation. In this case, there is a problem in that the signal transcription from the recording magnetic film is insufficient.
Furthermore, assuming that the magnetic domain is enlarged by utilizing the shrink operation or magnetic domain wall shift, when a conventional guide groove is used in association with a tracking servo, operation by the magnetic domain wall shift is prevented due to the influence of the guide groove, thereby reducing the amplitude of a reproduced signal. Alternatively, the influence of noise due to the groove causes a reduction in CNR upon reproduction of a signal. The above are also drawbacks.